Polyolefins like polyethylene (PE) and polypropylene (PP) cover more than 60% of the worldwide thermoplastic market, mainly due to their wide applications, including grocery bags, containers, toys, adhesives, home appliances, engineering plastics, automotive parts, medical applications and the like.
Porous polyolefins have a higher market value as they are very useful in the preparation of high impact rubber, which is used in the automobile industry. During preparation of high impact rubber, deposition of polymer inside the polypropylene resin is always preferred as compared to deposition on the resin surface as it makes the resin sticky and less movable, thus, interrupting plant operation. The porous morphology of polypropylene facilitates incorporation of polyethylene not only on the surface but also inside the pores which results in rubber formation with high impact resistance.
In order to meet the global demand of these polypropylene various catalysts such as Ziegler-Natta (ZN) catalysts, metallocene and the like have been used. Amongst these, Ziegler-Natta (ZN) catalysts are the preferred catalysts, mainly due to their capacity to produce high amounts of polyolefin at relatively low temperature and pressure.
However, for the preparation of polypropylene with porous morphology, Ziegler-Natta catalysts needs adequate control on polymerization kinetics to avoid formation of low particle size resins called fines.
Accordingly, several attempts were made in the past to prepare a Ziegler-Natta catalyst composition using different combinations of pro-catalysts, co-catalysts and external electron donors to obtain highly porous polymer resins.
However, it is observed that modification in the composition of the pro-catalyst containing a solid support and internal donors provides a degree of control on polymerization kinetics.
For instance, U.S. Pat. No. 6,395,670 suggests a catalyst for the polymerization of olefins. The catalyst comprises anhydrous magnesium dichloride, titanium compound and electron donors. Further, U.S. Pat. No. 6,395,670 suggests that the use of at least two electron donors in combination with anhydrous magnesium dichloride and titanium compound provides a balance between hydrogen response and isotacticity of the catalyst. The electron donors used in U.S. Pat. No. 6,395,670 include a compound containing two or more ether and/or ester groups.
U.S. Pat. No. 6,468,938 suggests pre-polymerized catalyst components for co-polymerization of olefins CH2═CHR. The catalyst components disclosed by U.S. Pat. No. 6,468,938 contain Ti, Mg, halogen and an electron donor compound, being capable of yielding, under standard polymerization conditions, a propylene homopolymer having insolubility in xylene at 25° C.
U.S. Pat. No. 8,222,357 suggests a pro-catalyst containing a multiple internal electron donor with at least two components, one of which is a silyl ester. Other components of the mixed internal electron donor include aromatic acid ester, di-ether and combinations thereof.
From the forgoing, it is observed that modifications in the pro-catalyst result in a Ziegler-Natta catalyst composition having lower polydispersity with adequate hydrogen response and controlled reaction kinetics. However, the aforementioned compositions failed to produce polymers having the desired melt flow index and porosity matrix required for producing high rubber polypropylene grades.
Therefore, there exists a need for a process for preparing a polymer having optimum pore size distribution and controlled porosity. There is also a need for a Ziegler-Natta catalyst composition having the required degree of control over the polymerization kinetics.